题目

This is a street crossing. There are red and green lights at each corner. Drivers must watch the lights carefully. When there is a green light, the cars may go. When there is a red light, the cars must stop. They must wait until the red light changes to green. Then they can go on. Sometimes the cars want to make a right turn or a left turn. They can make a right turn when the light is green or red. But they must wait until the green light is shining if they want to turn left. Some people are color-blind(色盲). They can not tell the difference between the red and green lights. These peo15．呼吸时，呼吸道之所以能够保证气流通畅的进出，主要是依靠构成呼吸道的（　　）A．鼻毛和纤毛B．毛细血管C．骨和软骨D．会厌软骨